Structure of molecular complexes formed in aqueous solutions of trifluoroacetic acid

Abstract

Optimized structures and vibration frequencies of H-bonded complexes formed by one or two molecules of CF3COOH (or CF3COO− ion) with water molecules and the monomers and dimers of formic, acetic, trifluoroacetic, and tribromoacetic acids were calculated using density functional theory (B3LYP/6–31++G(d,p)). The results were compared with available experimental data on the vibrational spectra and equilibrium composition of the CF3COOH-H2O system and the acids listed above. The structure of the hydrated forms of CF3COOH molecules and CF3COO− anions which are present in aqueous solutions of trifluoroacetic acid was determined. In these solutions at concentrations 100 % > [CF3COOH] > 35 %, (CF3COOH)2 ·(H2O)2 cyclic tetramers form in which the acid and water molecules are arranged pairwise side-by-side, and starting at 75 % CF3COOH, CF3COO−·(H2O)2 cyclic dihydrates of the anion are formed. The average strength of the hydrogen bonds in these heteroassociates is ∼9–10 kcal/mol, and the properties and mutual position of their constituent molecules almost completely determine their vibrational spectra in solution. This approach provides a good reproduction of the IR and Raman spectra of molecular complexes, that have a cyclic structure and an average H-bond energy not less than 7 kcal/mole, in solution.